The European eelpout is an unusual little fish, and one that few of us might ever expect to hear about. But it has been a curious subject of scientific study for many years, primarily because rather than laying eggs like most other fish, females give birth to live young. Exactly how female eelpouts manage to produce live offspring without an umbilical cord—an adaptation found only in placental mammals—remained one of the great mysteries of fish biology until recently, when Peter Skov, a researcher at the Technical University of Denmark, cracked the case. Skov and colleagues not only uncovered physiological adaptations in female eelpouts that support their unique reproductive strategy but also caught eelpout embryos in the act of suckling on follicles in the mother’s womb—arguably one of the most peculiar fish behaviors discovered of late.

A pregnant European eelpout. (Photo courtesy of Birgit Thorell)

The European eelpout, or Zoarces viviparus, is found from the White Sea to the British Isles and Baltic Sea to the southern parts of the English Channel, and according to Skov its long gestation period and reproductive strategy have raised important questions about the species’ physiology and evolution.

“The switch from being oviparous and spawning millions of eggs that require no further parental effort, to being viviparous and having internal fertilization of much fewer eggs (in the low hundreds) places some pretty large demands on the maternal organism,” Skov said. “She must now provide nutrition to all developing young, maintain adequate oxygen levels in the ovary, and ensure the removal of metabolic wastes produced by the embryos.”

X-ray negative of a pregnant eelpout. (Photo courtesy of Peter Skov)

Understanding how female eelpouts meet these demands has challenged scientists. As Skov explained, the womb environment is anything but amenable to the survival of live young. “Earlier work has repeatedly shown that the ovary is oxygen-depleted at levels that could not possibly sustain life,” he said. “At the same time, compositional analysis of the ovarian fluid has shown that it has no nutritional value.”

Skov’s discovery of viviparous adaptations in female eelpouts began with investigations of the fish’s ovary and follicles, capsule-like structures found in the ovary that surround and nourish egg cells. Using transmission electron microscopy of ovary and follicular tissue and three-dimensional casts of the follicles, Skov and his team performed detailed investigations of follicle anatomy, focusing in particular on the organization of blood vessels. This gave the researchers a better understanding of physical features that could influence the transfer of oxygen and nutrients from the follicle to the developing embryo.

Still, the eelpout viviparity puzzle was missing a piece, which Skov later realized through serendipitous discovery. “I needed to take some photographs of the internal surface of the ovary,” Skov explained. “I had a pregnant female that had been euthanized and fixed sitting in the refrigerator, so I removed the ovary, opened it, and started to remove the embryos. I noticed that some of the embryos appeared to be stuck and found this quite odd, so I put the ovary under the dissection microscope to see what was going on. And there they were, a handful of embryos had the follicle in their mouth, while the remaining 30–40 embryos were ‘loose’. This was a new observation!”

Skov believes that suckling in eelpouts had not been discovered previously because the embryos are easily disturbed, causing them to detach from the follicles prior to dissection, particularly in freshly killed animals. In his fixed specimen, however, some embryos had remained attached. “We subsequently found a protocol where we can make some of the embryos stay attached if we anaesthetize the female gradually and make sure she is ventilated while doing so,” he added.

The discovery of the suckling behavior fit with the team’s earlier observations concerning follicle structure. The abundance of blood vessels in the follicles influences gas transfer from the mother to the developing embryo. And although the ovary environment itself has generally low oxygen levels, the proximity of the follicle to the embryo’s gills enables oxygen to be exchanged very efficiently. In addition, the follicular fluid is relatively rich in proteins, and the structure of the follicle is such that these fluids can be readily transferred to a suckling embryo, thereby overcoming the problems presented by the otherwise low-nutrient environment of the womb.

The understanding of eelpout suckling provided by Skov’s study could fuel new research on viviparity. “If our paper spurs a renewed interest in viviparity in fishes that would be excellent,” he said. “I think probably some of the viviparous species where we already think we have a full understanding of reproduction and embryonic development deserve a second look.”

Skov also explained that the European eelpout has become an important sentinel, or indicator, species of global warming and pollution. “In terms of pollution, scientists are interested in looking at heavy metal accumulation and toxicants such as brominated flame retardants in eelpouts,” he said. “Some of these substances are accumulated in fatty tissues. So the question arises—if the female mobilizes her fatty reserves to provide nutrition for her young, are these substances then transferred to the embryos, making them pre-burdened at birth?”

With research on the environmental and biological effects of climate change and global warming on marine species intensifying, we may know the answer to this question sooner rather than later.

A regular Britannica Blog feature written by the encyclopedia’s own Kara Rogers, Science Up Front goes behind the headlines to bring researchers’ stories of discovery centerstage. Begun in 2009 to highlight the ingenious work of pioneering scientists and to bring greater accuracy to science reporting, Rogers goes straight to the source, exploring the latest advances in science, from medicine to nanotechnology to conservation, through first-hand interviews with researchers. The series covers all things science, so check back regularly to see who’s up on Science Up Front.